Landslides and damaged roads have made it impossible to reach the worst-affected communities in isolated areas.
Many people are thought to be living under canvas roofs in the rubble of damaged buildings.
The 7.5-magnitude quake rattled villages and a large gold mine in the country’s Enga province.
US seismologists said it struck 90km (56 miles) south of Porgera at 03:45 local time on Monday (17:45 GMT on Sunday).
There have been dozens of aftershocks, including a 6.0 quake on Sunday.
Local reports say dozens of people have been killed but there has been no official confirmation.
“The challenge is road access; it’s still not accessible to trucks and four-wheel drives,” Udaya Regmi, of the International Red Cross, told Reuters news agency.
“Big trucks cannot go there. It’s one of the reasons the food is becoming less and less. There are no fears of starvation yet but we’ve not got the full picture.”
Mr Regmi – who is based in the country – said that 147,000 people were in severe need of food, water and sanitation.
A state of emergency has been declared but the full scale of the damage will not be clear until relief workers can complete a full assessment.
“This was the biggest earthquake in 100 years and it spread 150km across the fault line,” he said.
“A number of urban settlements, as well as villages, have been affected, many in the form of landslides and landslips, which means that roads have been cut off, water contaminated and power knocked out.”
The governor of Hela province, Philip Undialu, told local media the damage was “extensive”.
“Our police station, courthouse, hospital… even private houses have been ripped apart or sunk into the ground,” Mr Undialu said from the country’s capital, Port Moresby.
Earthquakes are common in Papua New Guinea, which sits on the so-called Pacific Ring of Fire, a hotspot for seismic activity.
Part of the country’s northern coast was destroyed by a tsunami in 1998, caused by a 7.0 earthquake.
Some 2,200 people were killed. (Courtesy BBC)